The general use of air conditioning and refrigeration systems in vehicles in particular is well known. Descriptions of these systems can be found in many engineering textbooks and monographs such as W. F. Stoecker, Refrigeration and Air Conditioning McGraw-Hill, 1958.
These systems can be divided into two categories according to their principle of operation: (1) systems based on the compression of refrigerant vapor, and (2) systems based on the absorption of refrigerant vapor.
The most common refrigeration system is the compression system. The main components of a compression system are an evaporator, a compressor, a condenser and an expansion valve. In the operation of such a compression system, refrigeration is produced by the repeated process of compression, condensation, expansion and evaporation of a refrigerant, which is a liquid with a relatively high vapor pressure.
Two principal conditions must be satisfied for a compression system to operate: (1) energy must be supplied to operate the compressor; and (2) separate cooling must be provided for condenser operation. Because of these conditions, several disadvantages are inherent in the operation of a compression refrigeration system in vehicles.
Part of the engine power of the vehicle usually serves as the energy source for the compressor, thereby reducing the availability of power for locomotion and increasing fuel consumption. This drawback restricts application of the compression system mostly to relatively high-powered cars and industrial vehicles such as lorries. Moreover, the refrigeration effect is reduced when operating at low revolutions, and no refrigeration is generated at all when the engine stops.
The condenser is cooled by engine cooling water or by an air fan. Use of engine cooling water reduces engine efficiency and overloads the engine cooling system. Moreover, the refrigerants used in compression systems are usually freons which are environmentally harmful.
Despite these inherent disadvantages, compression refrigeration systems are rather common both in general use and in vehicles in particular, because their coefficient of performance ("C.O.P.") is high and construction is relatively simple. The C.O.P. is the ratio between the "cold" produced and the energy consumed.
The second type of refrigeration system is an absorption-separation ("absorption") system. Absorption systems have been known for a long time, their first use probably dates back to the American Civil War.
The operation principle of the absorption system is similar to that of the compression system, and its main components also include an evaporator and a condenser. However, instead of a compressor, which is a mechanical means for increasing the pressure difference between the evaporator and the condenser, the absorption system utilizes a generator filled with a mixture of at least two intermiscible substances (a refrigerant and an absorbent) and an absorber, in which the absorption of the refrigerant by the absorbent takes place. The refrigerant and the absorbent should have sufficiently different vapor pressures, so that when the generator is heated, the more volatile refrigerant evaporates to be condensed in the condenser.
The absorption system usually includes a pump to deliver the liquid from the absorber back to the generator. The vapor from the generator passes to the condenser, where it condenses into a liquid and then proceeds via a pressure reducing valve into the evaporator where it again turns into vapor with the accompanying cooling effect due to the absorption of its latent heat of vaporization.
The absorption system also includes fanned air or cooling liquid to cool the absorber and condenser. The main advantage of this system is that only a small amount of mechanical work is required for operation. Instead of expending a large quantity of mechanical energy to operate the compressor, only a negligible amount is needed to operate the pump and cooling means.
On the other hand, because the energy received by the system is in the form of heat supplied to the generator, the C.O.P. is much lower in absorption systems than in compression systems. Consequently, these systems are used in applications where power supply is not sufficient, but heat is available.
Another obstacle to using absorption systems is that the refrigerant-absorbent pair must possess a certain combination of properties: volatility, solubility, viscosity, latent heat of vaporization, etc. The common choices for the refrigerant-absorbent pair are water-ammonia and water-lithium bromide. However, these refrigerant-absorbent pairs operate with low C.O.P.'s when used in an absorption system.
Nevertheless, several have attempted to use absorption systems for refrigerating and air conditioning vehicles because of the very attractive possibility of utilizing the waste heat of exhaust gases as an external heat source for the generator. The systems described in U.S. Pat. Nos. 3,661,200 and 4,924,676, and British Application Nos. 1,368,911, and European Patent No. 350,764 are examples of these attempts.
One main disadvantage of these patented systems are associated with the problem of optimizing the conditions for transferring heat from the exhaust gases to the generator. For example, the absorption system described in U.S. Pat. No. 3,661,200 has a generator installed in a casing which extends along and over the exhaust conduit of the engine. U.S. Pat. No. 2,783,622 describes a helical generator which coils around the exhaust conduit.
The waste gases in these patented absorption systems usually enter the exhaust pipe at a temperature of between 600 and 800 degrees Celsius and a pressure of a few inches of Mercury; the gases move along the exhaust pipe at velocities of 60-100 meters/second. Effective extraction of heat from the exhaust pipe and its transfer under these conditions to a generator designed according to the above-mentioned patents requires a very large heat transfer area, which requires an excessively long generator and exhaust conduit. Such a configuration causes pumping and pressure drops.
For example, the invention of U.S. Pat. No. 2,783,622 ("'622 Patent") is not operative on a scale small enough to fit in a motor vehicle. Calculations derived from the disclosure of the system disclosed in the '622 Patent indicate that for a functional amount of heat transfer to occur, the exhaust bypass and the generator must each be 10 to 11 feet long. This is longer than many motor vehicles.
Furthermore, assuming that both a generator and exhaust bypass with sufficient area of heat transfer could fit under a motor vehicle, the diameter of the vapor lines in the absorption system of the '622 Patent would have to be more than 1 foot to provide for sufficient volumetric water vapor flow through the system to generate sufficient cooling.
The U.S. Pat. No. 3,661,200 ("'200 Patent") mentions the problem of ineffective heat transfer, but does not provide a specific solution. Additionally, the absorption system described in the '200 Patent operates under high pressure. Hence, that absorption system must have thick walls to withstand the relatively high pressure.
According to the '200 Patent, the interior wall of the generator serves as the exterior wall of the exhaust conduit along the area of heat transfer. ('200 Patent, col. 2, line 67 to col. 3, line 26). Hence, both the generator and the exhaust conduit must have thick walls. The system of the '200 Patent is similar to the system of the '622 Patent in that both systems must have an excessive heat transfer area to effectively operate. Hence, the configuration of the '200 Patent which requires both an exhaust conduit and a generator with long, thick bulky walls would be difficult to install in a motor vehicle.
Some patented absorption systems relate to an alternative configuration where the generator is placed inside the exhaust pipe to ensure direct contact with the hot waste gases. The presence of a generator in the exhaust pipe causes back pressure on the engine outlet ports which reduces engine thermodynamic efficiency.
As an inevitable consequence of all the stated problems, the known refrigeration and air conditioning absorption systems for vehicles become complex, bulky and inefficient. A practical, efficient and economical absorption system based on the utilization of waste heat from exhaust gases that does not adversely affect the engine performance is still unavailable.